1. Field of the Invention
The present invention relates to a projection display device of the type in which separate color images are combined and projected as of an enlarged picture image.
2. Description of Related Art
A known type of projection display device consists of a liquid crystal color projector wherein R (red), G (green) and B (blue) illuminating light rays are applied to three liquid crystal panels which give R, G and B color separation information, respectively, to obtain separate R, G and B images. The images are then combined and projected as an enlarged color image through a projection lens.
FIG. 4 is a schematic view of a conventional liquid crystal color project or which employs a dichroic prism and dichroic mirrors for separation and combination (synthesis) of light rays, as described above. Liquid crystal panels 12, 13 and 14 for R, G and B are disposed in opposing relation to three surfaces (defined as incidence surfaces), respectively, of a dichroic prism 11 which is in the shape of a rectangular parallelepiped. A projection lens 15 is disposed so as to face the remaining surface (defined as an emergence surface E) of the prism. A R (red) reflecting dichroic mirror 12a, a G (green) reflecting dichroic mirror 13a and a B (blue) reflecting dichroic mirror 14a are disposed at the respective incidence sides of the liquid crystal panels 12, 13 and 14 at 45.degree. to the corresponding optical axes. Illuminating light rays are directed onto these dichroic mirrors 12, 13 and 14. The illuminating light rays are obtained by passing light from a white light source 16 through a collimating lens (condenser lens) 17 and thereby producing parallel rays of light. The parallel light rays are applied successively to the B reflecting dichroic mirror 14a, a total reflecting mirror 18, the G reflecting dichroic mirror 13a, a total reflecting mirror 19 and the R reflecting dichroic mirror 12a as shown in FIG. 4.
The dichroic prism 11 is provided with a B (blue) reflecting film 11a and a R (red) reflecting film 11b which are positioned at 45.degree. to the corresponding optical axes and which are perpendicular to each other. Both the reflecting films 11a and 11b transmit G (green) light.
In the liquid crystal color projector, when the liquid crystal panels 12, 13 and 14 are irradiated with parallel light rays produced by the white light source 16 and the collimating lens 17 while these panels 12, 13 and 14 are being switched on the basis of the corresponding color information, red, green and blue light rays respectively transmitted by the liquid crystal panels 12, 13 and 14 enter the dichroic prism 11. The incident light rays are combined (superposed) through the B and R reflecting films 11a and 11b provided in the prism 11, and the combined picture image emerges from the emergence surface E of the prism 11 and is projected in the form of an enlarged image on a screen 20 through the projection lens 15. The reference numeral 22 denotes an aperature (diaphragm) of the projection lens 15.
In this liquid crystal color projector, parallel rays are employed as illuminating light so as to enable the dichroic mirrors and prism to perform optimally. A telecentric lens is generally employed as the projection lens 15 to avoid color shifting due to an incident angle difference.
However, in the above-described conventional projection display device employing a dichroic prism and dichroic mirrors the contrast of the image projected on the screen 20 is low.
The inventors of the present invention investigated the causes of the low contrast projected image in the prior art and, as a result, have found that in the projection display device employing the dichroic prism 11 the respective outer surfaces of the dichroic prism 11 and the liquid crystal panels 12, 13 and 14 are defined by flat surfaces which are parallel to each other. The resulting surface reflection that occurs at the dichroic prism, results in a low contrast image.
FIG. 5 illustrates the cause of low contrast of the projected image in the conventional display devices. Assuming that the upper and lower light rays of a bundle of rays (shown by the solid line) emerging from the point A on the liquid crystal panel 12 (13, 14) are a1 and a2, respectively. The reflected light (shown by the dotted line) of a2 at the emergence surface E (the end surface closer to the screen 20) of the dichroic prism 11 reaches the point B on the liquid crystal panel 12 and is then reflected again therefrom. The bundle of rays passing through the dichroic prism 11, the projection lens 15 and the aperture 22 reaches the screen 20 as shown by the arrow A'. On the other hand, the upper and lower rays b1 and b2 of another bundle of rays (shown by the one-dot chain line) emerging from the point B on the liquid crystal panel 12 pass through the same optical paths as those shown by the dotted lines and reach the screen 20 as shown by the arrow B'. Thus, it is clear that the light rays shown by the dotted lines and one-dot chain lines coincide with each other in a telecentric optical system, although in the figure the dotted and one-dot chain lines are drawn separately from each other for sake of the clarity. Accordingly, the reflected light rays shown by the dotted lines cause a ghost image, which causes a low contrast in the projected image.
Since the low contrast is caused by the surface reflection as described above, it can theoretically be prevented by providing antireflection films on the dichroic prism 11 and the liquid crystal panel 12, respectively. However, it is impossible to obtain perfect antireflection films and employment of antireflection films leads to an increase in the production cost. The problem of low contrast may be solved to a certain extent by interposing a liquid having a specific refractive index between the emergence surface E of the dichroic prism 11 and the projection lens 15, using the technique proposed in Japanese Unexamined Patent Publication No. 62-208016 in regard to a color projector comprising a CRT. However, provision of a liquid having a specific refractive index in a space will complicate the arrangement of the device and also cause an increase in the cost.